1. Field of the Invention
This invention relates to coupling any one or many wavelengths into an optical output port, and in particular, to combining plural multi-wavelength laser beams into a single mode fiber.
2. Description of Related Art
When a plurality of light sources is to be connected to fewer optical ports, the mechanical interconnection and alignment of the optical elements become increasingly difficult with an increase in the number of light sources and/or a decrease in a number of optical output ports. The physical dimensions of the light-emitting devices limit the spacing of beams produced by the light-emitting devices such that physically juxtaposing the light-emitting devices would result in the light beams being produced in an area that is larger than the optical cross-section of an optical fiber that can practically be used to implement the optical output port.
Thus, the problem of interconnecting a plurality of light beams produced by a plurality of light-emitting devices is reduced to the need to direct the plurality of light beams into an area that is less than or equal to the optical cross-section of the optical output channel. A passive star coupler, which operates on the concept of interference or wavefront division, can be used to provide this interconnection. However, the average power at each of the output ports of the star is equal to only 1/N of the incident power of that channel, where N is the number of beams being combined. This loss is due to splitting which may not be efficiently recovered by an amplifier at either the input port or output port unless the saturation power of the amplifier is increased to be substantially N times the value of the power transmitted, which degrades the signal-to-noise ratio.
Another way to combine light beams into one optical output port is in a free space fashion using narrow bandpass optical filters or using diffractive elements. Such filters and diffractive elements do not have the 1/N loss inherent in the star coupler approach. Rather, the power of each laser with its own wavelength is combined in an additive fashion to be transmitted in a single fiber. However, more recent systems use multi-wavelength laser arrays, in which each laser may be wavelength tuned. This tunability brings wavelength agility to the systems, providing redundancy and error protection. However, when each light source can output multiple wavelengths, rather than just one specified wavelength, neither optical filters nor diffractive elements, which are both highly wavelength dependent, can be used for combining. In other words, the optical filter or diffractive element which is suitable for one wavelength is not suitable for other wavelengths. This would seemingly force the use of the star couplers with their inherent loss and subsequent need for optical amplifiers as the only method for coupling multiple wavelength beams at a useable power level.
Another method for coupling an array of light sources into a single fiber includes steering laser beams to a single focusing lens. This steering brings the light beams closer together. However, in current applications of steering, all the light sources are to have the same stable output and the coupling is merely for increasing the optical power, not for any type of multiplexing. Further, the light sources used are extremely collimated gas lasers, not presenting the difficulties of the highly divergent semiconductor light sources. Finally, the array of light beams output to the lens has the same configuration as the array of light sources, making its application for large linear arrays preferred in multiplexing applications impractical.
The present invention is therefore directed to combining an array of light sources, the light sources having different wavelengths from one another and/or the light sources having variable wavelengths, into an optical output port which substantially overcomes one or more of the problems due to the limitations and disadvantages of the related art.
These and other advantages may be realized in accordance with the present invention by using a free space propagation approach which does not have the inherent 1/N loss of the integrated star couplers. The free space propagation approach of the present invention uses spatial wavefront segmentation rather than wavefront splitting used by the beamsplitters. This allows any light beams at any wavelength to be combined uniformly into a single fiber. Further, it is an object of the present invention to create such a free space propagation system which is wavelength insensitive.
These and other objects of the present invention will become more readily apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating the preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.